Cytosolic calcium elevation in response to Fc receptor cross-linking in undifferentiated and differentiated U937 cells

Historical Overview of Store-Operated Ca(2+) Entry

Calcium influx is an essential mechanism for the activation of cellular functions both in excitable and non-excitable cells. In non-excitable cells, activation of phospholipase C by occupation of G protein-coupled receptors leads to the generation of inositol 1,4,5-trisphosphate (IP3) and diacylglycerol (DAG), which, in turn, initiate two Ca(2+) entry pathways: Ca(2+) release from intracellular Ca(2+) stores, signaled by IP3, leads to the activation of store-operated Ca(2+) entry (SOCE); on the other hand, DAG activates a distinct second messenger-operated pathway. SOCE is regulated by the filling state of the intracellular calcium stores. The search for the molecular components of SOCE has identified the stromal interaction molecule 1 (STIM1) as the Ca(2+) sensor in the endoplasmic reticulum and Orai1 as a store-operated channel (SOC) subunit. Furthermore, a number of reports have revealed that several members of the TRPC family of channels also take part of the SOC macromolecular complex. This introductory chapter summarizes the early pieces of evidence that led to the concept of SOCE and the components of the store-operated signaling pathway.

The role of calcium signaling in phagocytosis

Immune cells kill microbes by engulfing them in a membrane-enclosed compartment, the phagosome. Phagocytosis is initiated when foreign particles bind to receptors on the membrane of phagocytes. The best-studied phagocytic receptors, those for Igs (FcgammaR) and for complement proteins (CR), activate PLC and PLD, resulting in the intracellular production of the Ca(2+)-mobilizing second messengers InsP3 and S1P, respectively. The ensuing release of Ca(2+) from the ER activates SOCE channels in the plasma and/or phagosomal membrane, leading to sustained or oscillatory elevations in cytosolic Ca(2+) concentration. Cytosolic Ca(2+) elevations are required for efficient ingestion of foreign particles by some, but not all, phagocytic receptors and stringently control the subsequent steps involved in the maturation of phagosomes. Ca(2+) is required for the solubilization of the actin meshwork that surrounds nascent phagosomes, for the fusion of phagosomes with granules containing lytic enzymes, and for the assembly and activation of the superoxide-generating NADPH oxidase complex. Furthermore, Ca(2+) entry only occurs at physiological voltages and therefore, requires the activity of proton channels that counteract the depolarizing action of the phagocytic oxidase. The molecules that mediate Ca(2+) ion flux across the phagosomal membrane are still unknown but likely include the ubiquitous SOCE channels and possibly other types of Ca(2+) channels such as LGCC and VGCC. Understanding the molecular basis of the Ca(2+) signals that control phagocytosis might provide new, therapeutic tools against pathogens that subvert phagocytic killing.

Single-cell immunosensors for protein detection

A single-cell detector is described that combines the natural signal amplification of whole-cell biosensors with the flexibility and specificity of immunological recognition. An immune cell that expresses receptors for the constant region of immunoglobulin G (IgG) is loaded with a Ca(2+)-indicating dye and with antibodies directed against the protein of interest. Introduction of a multivalent protein antigen causes cross-linking of the receptors, which results in a detectable increase in the concentration of cytosolic Ca(2+). Some immune cell lines respond to stimulation with oscillations in their cytosolic Ca(2+) levels that complicate their use as detectors. The human monocytic cell line U-937, when treated with the cytokine interferon-gamma, produces a large, short-lived Ca(2+) signal in response to cross-linking of its high-affinity IgG receptors. U-937 was therefore chosen for development as an immunity-based detector. Human and rabbit antibodies are found to effectively stimulate the cell, causing a prompt and transient response. The cell is able to respond to repeated stimulation, though the response diminishes during rapid stimulation. Ovalbumin can be detected in micromolar concentrations. Possible fundamental constraints on the size of a detectable analyte are discussed.

Microfluidic device for single-cell analysis

We have developed a novel microfluidic device constructed from poly(dimethylsiloxane) using multilayer soft lithography technology for the analysis of single cells. The microfluidic network enables the passive and gentle separation of a single cell from the bulk cell suspension, and integrated valves and pumps enable the precise delivery of nanoliter volumes of reagents to that cell. Various applications are demonstrated, including cell viability assays, ionophore-mediated intracellular Ca2+ flux measurements, and multistep receptor-mediated Ca2+ measurements. These assays, and others, are achieved with significant improvements in reagent consumption, analysis time, and temporal resolution over macroscale alternatives.

Differentiation of the human monocyte cell line, U937, with dibutyryl cyclicAMP induces the expression of the inhibitory Fc receptor, FcgammaRIIb

FC receptor for IgG receptor (Fcgamma) mediated activation of macrophages is essential for the clearance of immune complexes and control of inflammation. However, activated macrophages play an integral role in tissue destruction associated with autoimmune and inflammatory disease processes. Understanding the mechanisms which balance activating and inhibitory signals generated by immune complexes are therefore of critical importance to human disease. Here, we demonstrate that differentiation of the human monocytic U937 cell line to a macrophage phenotype with dibutyryl cyclicAMP induces both mRNA and protein expression of the inhibitory IgG receptor, FcgammaRIIb1. We further demonstrate that, following receptor aggregation, FcgammaRII transiently recruits the 5'-inositol phosphatase, SHIP. These data define a role for FcgammaRIIb in the modulation of immune complex mediated macrophage activation in a human model system.

Differential recruitment of accessory molecules by FcgammaRI during monocyte differentiation

Aggregation of the human high-affinity receptor for immunoglobulin G, FcgammaRI, results in initiation of intracellular signaling cascades. However, as the receptor contains no known signaling motif, it is required to recruit an accessory molecule. The gamma chain has been proposed to fulfil this role. Here, we show that in U937 cells differentiated to a more macrophage-like phenotype with dibutyryl cAMP, FcgammaRI no longer signals through the gamma chain but rather uses FcgammaRIIa to initiate tyrosine phosphorylation. Expression of the gamma chain is, however, increased in the dbcAMP-induced cells, but here the gamma chain specifically associates with the IgA receptor, FcalphaRI. Recruitment of the gamma chain either by FcgammaRI in cytokine-primed cells or by FcalphaRI in dbcAMP-induced cells couples ligand binding to the activation of phosphatidyl choline-specific phospholipase D.

Endocytosis of FcgammaRI is regulated by two distinct signalling pathways

Aggregation by immune complexes of receptors specific for the Fc region of IgG results in their internalisation and disposal by trafficking to lysosomes. We show here that internalisation of FcgammaRI by IFN-gamma treated U937 cells following receptor aggregation by cross-linking antibodies requires the activation of two distinct signalling pathways. The pathways were functionally dissected in streptolysin-O-permeabilised cells by capitalising on their relative dependence on active GTP binding proteins. One pathway required the presence of GTP-gammaS or active betagamma subunits, the other did not. Use of inhibitors revealed that the betagamma-independent pathway required activation of PI 3-kinases and was PKC-independent In contrast, the betagamma-dependent pathway involved activation of phospholipase C-beta and PKC, but was PI 3-kinase-independent. Both these pathways were found to be active in intact cells and are likely to determine receptor trafficking following internalisation.

The sialoadhesin CD33 is a myeloid-specific inhibitory receptor

Activating and inhibitory receptors act in concert to regulate cellular activation. Inhibitory receptors are characterized by the presence of a characteristic sequence known as an immunoreceptor tyrosine-based inhibitory motif (ITIM) in their cytoplasmic tail. Phosphorylated ITIM serve as docking sites for the SH2-containing phosphatases which then inhibit signal transduction. CD33 is a member of the immunoglobulin superfamily and contains two immunoglobulin-like domains, a transmembrane region and a cytoplasmic tail that has two potential ITIM sequences. CD33 expression is restricted to cells of myelomonocytic lineage. The precise function of CD33 is unknown although it is a lectin that binds sialic acid residues in N- and O-glycans on cell surfaces. Co-immunoprecipitation studies demonstrate that CD33 associates with the SH2-containing tyrosine phosphatase SHP-1 in monocytes. The proximal ITIM is necessary and sufficient for SHP-1 binding which is mediated by the aminoterminal SH2 domain. Treatment of SHP-1 with a phosphopeptide representing the proximal CD33 ITIM results in increased SHP-1 enzymatic activity. CD33 exerts an inhibitory effect on tyrosine phosphorylation and Ca(2+) mobilization when co-engaged with the activating FcgammaRI receptor. This data indicates that CD33 is an inhibitory receptor that may regulate FcgammaRI signal transduction.

FcgammaRI activation of phospholipase Cgamma1 and protein kinase C in dibutyryl cAMP-differentiated U937 cells is dependent solely on the tyrosine-kinase activated form of phosphatidylinositol-3-kinase

The human high affinity receptor for immunoglobulin G, FcgammaRI, in dibutyryl cyclic AMP (dbcAMP)-differentiated U937 cells, is coupled to the activation of phospholipase C (PLC) and the conventional protein kinase C (PKC) isoforms, alpha, beta, and gamma. Here we demonstrate that aggregation of FcgammaRI activates the tyrosine-kinase regulated form of phosphatidylinositol-3-kinase (PI-3-kinase) and that an increase of phosphatidylinositol trisphosphate (PIP3) is essential for the activation and translocation of PLCgamma1 in these cells. In addition, activation of the PKC isoforms was ablated by specific inhibitors of PI3-kinase or by overexpression of a dominant negative p85 subunit of PI3-kinase. The findings reported here demonstrate that PLCgamma1 and PKC activation by FcgammaRI are downstream of PI3-kinase, and that in contrast to cytokine primed cells, only the tyrosine-kinase activated isoform of PI3-kinase is coupled to FcgammaRI in dbcAMP-differentiated cells.

The human high-affinity immunoglobulin G receptor activates SH2-containing inositol phosphatase (SHIP)

On cytokine-primed U937 cells, aggregation of the human high-affinity immunoglobulin receptor, FcgammaRI, initiates signal transduction cascades which lead to the release of calcium from intracellular stores and no significant calcium influx. In these cells, aggregation of FcgammaRI results in no significant increase in inositol trisphosphate production, but rather phospholipase D is activated. Here we show that, in interferon-gamma (IFN-gamma)-primed cells, the SH2 containing inositol 5' phosphatase, SHIP, is constitutively associated with the membrane fraction. Following aggregation of FcgammaRI, SHIP is rapidly and transiently tyrosine phosphorylated and becomes associated with the adapter molecule Shc. Shc also becomes tyrosine phosphorylated and translocates from the cytoplasm to the membrane fraction concomitant with the association between Shc and SHIP. Further, SHIP and Shc appear to be recruited to membrane-associated immune complexes following FcgammaRI aggregation. As no immunoreceptor inhibitory motif has been demonstrated to associate with FcgammaRI, these data suggest that SHIP may be recruited to the receptor through an SH2 domain interaction with Shc.

Differentiation-dependent switch in protein kinase C isoenzyme activation by FcgammaRI, the human high-affinity receptor for immunoglobulin G

Aggregation of receptors for the constant region (Fc) of immunoglobulin G on myeloid cells results in endocytosis or phagocytosis and cellular activation. Previous work has shown, using the cell line U937, that the high-affinity immunoglobulin G receptor, FcgammaRI, activates alternate intracellular signalling pathways depending on the cell differentiation state, which results in a marked change in the nature of calcium transients within the cell. Here, we show that protein kinase C (PKC) is activated in both interferon-gamma (IFN-gamma) -primed and dibutyryl cyclic AMP (dbcAMP) -differentiated cells but that the nature of the particular isoenzymes recruited differs. Thus, in IFN-gamma-primed U937 cells, FcgammaRI aggregation results in an increase of PKC activity which is essentially calcium independent resulting from the translocation to the membrane of the novel PKCs, delta and epsilon, together with the atypical PKC zeta. However, in cells differentiated to a more macrophage phenotype, all PKC enzyme activity after receptor aggregation is calcium dependent. Consistent with this finding, the isoenzymes translocated to the nuclear-free membrane fraction are the conventional PKCs alpha, beta and gamma; results consistent with our previous finding that FcgammaRI couples to phospholipase C in such dbcAMP-differentiated cells. Thus, the nature of PKC isoenzyme activated following FcgammaRI aggregation is defined by differentiation. The calcium dependence of the PKC isoenzyme is consistent with the duration of calcium transients previously reported in the two differentiation states.

Interferon-gamma elicits a G-protein-dependent Ca2+ signal in human neutrophils after depletion of intracellular Ca2+ stores

Interferon-gamma (IFN-gamma) has multiple effects on Ca2+ signalling in polymorphonuclear neutrophils (PMNs), including evoked cytosolic Ca2+ transients, increased capacitative calcium influx and increased sequestration of Ca2+ in intracellular stores. The present study was conducted to elucidate the mechanism behind the Ca2+ transients. As observed before, the IFN-gamma-evoked Ca2+ signals were apparent when extracellular Ca2+ was removed. A new finding was that the proportion of responding cells and the extent of calcium release increased with increasing time in EGTA buffer. As assessed by N-formyl-methionyl-leucyl-phenylalanine (fMLP)-stimulated Ca2+ release, the intracellular stores were depleted during this incubation period, and the extent of depletion correlated well with the appearance of IFN-gamma-induced Ca2+ signals. This store dependence of the IFN-gamma-induced Ca2+ signals was confirmed by the appearance of IFN-gamma-evoked Ca2+ signals in the presence of extracellular Ca2+ after store depletion by thapsigargin. The appearance of IFN-gamma-mediated Ca2+-signals in the presence of EGTA indicates that IFN-gamma stimulates Ca2+ release from intracellular stores. This was confirmed by the inability of the calcium transportation blocker La3+ to abolish the IFN-gamma response and the total abrogation of the response by the phospholipase C inhibitor U73122. Although these latter results imply a role for inositol 1,4,5-trisphosphate(IP3) in IFN-gamma signalling, comparison of IFN-gamma-evoked responses with fMLP responses revealed clear differences that suggest different signal-transduction pathways. However, responses to fMLP and IFN-gamma were both depressed by pertussis toxin, and the IFN-gamma responses were, in addition, inhibited by the tyrosine kinase inhibitor genistein. Further evidence of the involvement of tyrosine kinase was a slight stimulatory effect of the protein tyrosine phosphatase inhibitor sodium orthovanadate. The PI-3K activity was of minor importance. In conclusion, we present evidence of a novel signal-transduction mechanism for IFN-gamma in PMNs, dependent on tyrosine kinase activity, a pertussis toxin-sensitive G protein and phospholipase C activity.

Lysosomal routing of Fc gamma RI from early endosomes requires recruitment of tyrosine kinases

The high-affinity receptor for immunoglobulin G (Fc gamma RI) plays a central role in the clearance of immune complexes by mediating their internalization and delivery to lysosomes. In monocytic U937 cells, receptor internalization is independent of tyrosine kinase activity. However, the tyrosine kinase inhibitor, genistein, prevents further progress of the receptor to lysosomes and traps it in a sub-plasma membrane early endosome. Similarly, Fc gamma RI expressed in COS cells is able to internalize immune complexes but is unable to translocate to lysosomes. This suggests that Fc gamma RI, whose cytoplasmic tail is devoid of known signalling motifs, must recruit tyrosine kinases via its gamma-chain to achieve lysosomal delivery. We show that a chimera of the extracellular domain of Fc gamma RI and the cytoplasmic tail of the gamma-chain is both internalized and efficiently trafficked to lysosomes. Our study suggests that a key function of the gamma-chain is recruitment of tyrosine kinases to initiate the intracellular signalling pathways required to target Fc gamma RI following immune complex aggregation to lysosomes and not to initiate endocytosis per se.

FcgammaRI coupling to phospholipase D initiates sphingosine kinase-mediated calcium mobilization and vesicular trafficking

Aggregation of receptors specific for the constant region of immunoglobulin G activates a repertoire of monocyte responses that can lead ultimately to targeted cell killing via antibody-directed cellular cytotoxicity. The high affinity receptor, FcgammaRI, contains no recognized signaling motif in its cytoplasmic tail but rather utilizes the gamma-chain of FcepsilonRI as an accessory molecule to recruit tyrosine kinases for signal transduction. We show here that, in a human monocytic cell line primed with interferon-gamma, FcgammaRI mobilizes intracellular calcium stores using a novel pathway that involves tyrosine kinase coupling to phospholipase D and resultant downstream activation of sphingosine kinase. Moreover, FcgammaRI is not coupled to phospholipase C; hence, calcium release from intracellular stores occurred in the absence of any measurable rise in inositol triphosphate. Finally, as this novel activation pathway is also shown to be responsible for mediating the vesicular trafficking of internalized immune complexes for degradation, it is likely to play a key role in controlling intracellular events triggered by FcgammaRI.

Aggregation of the human high affinity immunoglobulin G receptor (FcgammaRI) activates both tyrosine kinase and G protein-coupled phosphoinositide 3-kinase isoforms

Phosphoinositide 3-kinases (PI3-kinases) play an important role in the generation of lipid second messengers and the transduction of a myriad of biological responses. Distinct isoforms have been shown to be exclusively activated either by tyrosine kinase-coupled or G protein-coupled receptors. We show here, however, that certain nonclassical receptors can couple to both tyrosine kinase- and G protein-dependent isoforms of PI3-kinase: thus, aggregation of FcgammaRI, the human high affinity IgG receptor, on monocytes unusually leads to activation of both of these types of PI3-kinase. After aggregation of FcgammaRI, phosphatidylinositol 3,4, 5-triphosphate (PIP3) levels rise rapidly in interferon gamma-primed cells, reaching a peak within 30 sec. Moreover, and in contrast to the situation observed after stimulation of these cells with either insulin or ATP, which exclusively activate the tyrosine kinase- and G protein-coupled forms of PI3-kinase, respectively, PIP3 levels remain elevated up to 15 min after receptor aggregation. We show here that although the initial peak results from transient activation of the p85-dependent p110 isoform of PI-3kinase, presumably through recruitment of tyrosine kinases by the gamma chain, the later sustained rise of PIP3 results from activation of the G protein betagamma subunit-sensitive isoform, p110gamma. This finding indicates that receptors lacking an intrinsic signaling motif, such as FcgammaRI, can recruit both tyrosine kinase and G protein-coupled intracellular signaling molecules and thereby initiate cellular responses.

A molecular switch changes the signalling pathway used by the Fc gamma RI antibody receptor to mobilise calcium

BACKGROUND

Leukocytes express Fc gamma receptors, which are specific for the constant region of immunoglobulin G. Aggregation of these receptors activates a repertoire of responses that can lead to targeted cell killing by antibody-directed cellular cytotoxicity. The nature of the myeloid response to Fc gamma receptor aggregation is highly variable and depends on the maturation state of the cell, but little is known about the signalling mechanisms underlying this variability.

RESULTS

We show here that differentiation of a monocytic cell line, U937, to a more macrophage phenotype resulted in an absolute and fundamental switch in the nature of the phospholipid signalling pathway recruited following Fc gamma receptor aggregation. In cytokine-primed monocytes, aggregation of the high-affinity receptor Fc gamma RI resulted in the activation of phospholipase D and sphingosine kinase, which in turn led to the transient release of stored calcium; these effects were mediated by the gamma chain, an Fc gamma RI accessory protein. In contrast, in cells differentiated to a more macrophage type, aggregation of Fc gamma RI resulted in the Fc gamma RIIa-mediated activation of phospholipase C, and the resulting calcium response was prolonged as calcium entry was stimulated.

CONCLUSIONS

The switch in Fc gamma RI signalling pathways upon monocyte differentiation is mediated by a switch in the accessory molecule recruited by Fc gamma RI, which lacks its own intrinsic signal transduction motif. As many immune receptors have separate polypeptide chains for ligand binding and signal transduction (allowing a similar switch in signalling pathways), the mechanism described here is likely to be widely used.

Spatial and temporal separation of calcium signals in myeloid cells stimulated by immune complexes

Stimulation of large (100 microns) human myeloid cells with immune complexes resulted in Ca2+ spiking. Both global and regional changes in the intracellular cytosolic free Ca2+ concentration were detected in response to immune complex stimulation. The regional changes were mediated by release of Ca2+ from stores, whereas global changes were mediated by Ca2+ influx. They occurred independently of each other, with release of Ca2+ from intracellular Ca2+ stores being separated from transmembrane influx of Ca2+. Bromophenacyl bromide, an 1-plastin binding agent, inhibited store release without preventing transmembrane influx of Ca2+. The large size of the myeloid cells used here allowed the visualisation of the spatial and temporal separation of store release from transmembrane influx of Ca2+, providing further evidence for the existence of independent Ca2+ store release and Ca2+ influx mechanisms in these cells.

A novel inhibitory action of wheat germ agglutinin on phospholipase C in HEL and MEG-01 cell lines

Stimulation of HEL megakaryocytic cells by Fc gammaRIIA crosslinking is associated with tyrosine phosphorylation of syk and phospholipase C gamma2 (PLCgamma2) and is accompanied by formation of inositol phosphates and release of intracellular Ca2+. These responses are inhibited by the kinase inhibitors, staurosporine and ST271. In contrast, the G-protein receptor agonist, thrombin induces formation of inositol phosphates and release of intracellular calcium without an increase in tyrosine phosphorylation. The plant lectin wheat germ agglutinin (WGA) stimulates tyrosine phosphorylation of syk and PLCgamma2 but surprisingly does not stimulate formation of inositol phosphates and induce release of intracellular Ca2+. WGA also inhibited formation of inositol phosphates and release of intracellular Ca2+ by Fc gammaRIIA crosslinking and thrombin-stimulation. A similar inhibitory effect of WGA was observed against elevation of Ca2+ by the same two stimuli in MEG-01 megakaryotic cells. The results demonstrate a novel pathway of inhibition of PLC on crosslinking of cell surface proteins that is not present in platelets.

Engagement of the Lewis X Antigen (CD15) Results in Monocyte Activation

We previously reported that monocyte adhesion to tumor necrosis factor-α (TNF-α)–treated endothelial cells increased expression of tissue factor and CD36 on monocytes. Using immunological cross-linking to mimic receptor engagement by natural ligands, we now show that CD15 (Lewis X), a monocyte counter-receptor for endothelial selectins may participate in this response. We used cytokine production as a readout for monocyte activation and found that CD15 cross-linking induced TNF-α release from peripheral blood monocytes and cells from the monocytic cell line MM6. Quantitative reverse transcriptase-polymerase chain reaction (RT-PCR) showed an increase in steady-state TNF-α mRNA after 3 to 4 hours of cross-linking. CD15 cross-linking also concomitantly increased interleukin-1β (IL-1β) mRNA, while no apparent change was observed in the levels of β-actin mRNA, indicating specificity. To examine transcriptional regulation of cytokine genes by CD15 engagement, a CAT plasmid reporter construct containing IL-1β promoter/enhancer sequences was introduced into MM6. Subsequent cross-linking of CD15 increased CAT activity. CD15 engagement by monoclonal antibody also attenuated IL-1β transcript degradation, demonstrating that signaling via CD15 also had posttranscriptional effects. Nuclear extracts of anti-CD15 cross-linked cells demonstrated enhanced levels of the transcriptional factor activator protein-1, minimally changed nuclear factor-κB, and did not affect SV40 promoter specific protein-1. We conclude that engagement of CD15 on monocytes results in monocyte activation. In addition to its well-recognized adhesive role, CD15 may function as an important signaling molecule capable of initiating proinflammatory events in monocytes that come into contact with activated endothelium.

Engagement of the Lewis X Antigen (CD15) Results in Monocyte Activation

We previously reported that monocyte adhesion to tumor necrosis factor-α (TNF-α)–treated endothelial cells increased expression of tissue factor and CD36 on monocytes. Using immunological cross-linking to mimic receptor engagement by natural ligands, we now show that CD15 (Lewis X), a monocyte counter-receptor for endothelial selectins may participate in this response. We used cytokine production as a readout for monocyte activation and found that CD15 cross-linking induced TNF-α release from peripheral blood monocytes and cells from the monocytic cell line MM6. Quantitative reverse transcriptase-polymerase chain reaction (RT-PCR) showed an increase in steady-state TNF-α mRNA after 3 to 4 hours of cross-linking. CD15 cross-linking also concomitantly increased interleukin-1β (IL-1β) mRNA, while no apparent change was observed in the levels of β-actin mRNA, indicating specificity. To examine transcriptional regulation of cytokine genes by CD15 engagement, a CAT plasmid reporter construct containing IL-1β promoter/enhancer sequences was introduced into MM6. Subsequent cross-linking of CD15 increased CAT activity. CD15 engagement by monoclonal antibody also attenuated IL-1β transcript degradation, demonstrating that signaling via CD15 also had posttranscriptional effects. Nuclear extracts of anti-CD15 cross-linked cells demonstrated enhanced levels of the transcriptional factor activator protein-1, minimally changed nuclear factor-κB, and did not affect SV40 promoter specific protein-1. We conclude that engagement of CD15 on monocytes results in monocyte activation. In addition to its well-recognized adhesive role, CD15 may function as an important signaling molecule capable of initiating proinflammatory events in monocytes that come into contact with activated endothelium.

IgG-induced Ca2+ oscillations in differentiated U937 cells; a study using laser scanning confocal microscopy and co-loaded fluo-3 and fura-red fluorescent probes

We have investigated, at the single cell level, intracellular Ca2+ ([Ca2+]i) modulations triggered by the high affinity receptor for IgG, Fc gamma RI, in the monocytic cell line, U937. Cells were co-loaded with the Ca(2+)-sensitive dyes, Fluo-3 and Fura-Red, by incubation with their acetoxymethyl (AM) esters and confocal ratio imaging was used to monitor the [Ca2+]i changes induced by antibody cross-linking of IgG-loaded Fc gamma RI. A single Ca2+ spike was observed in 81% of untreated cells whereas dibutyryl cAMP-induced differentiation into a more macrophage cell type resulted in a sub-population of cells (44%) responding to receptor cross-linking with calcium oscillations. This change in calcium signalling may explain the difference in functional responses triggered by Fc gamma RI in monocytes and macrophages. Analysis of the Fluo-3 and Fura-Red fluorescence, after AM-ester loading, showed that both dyes have similar photobleach rates and intracellular localization allowing compensation for shifts in focal plane, dye photobleaching and non-uniformity of dye loading. In addition, because the binding kinetics of both dyes are equivalent, accurate temporal information can be gained about [Ca2+] changes. There are, however, two major problems with this dual indicator technique. Firstly, loading from AM esters results in considerable variation between cells in the intracellular concentration ratio of the two dyes, making calibration difficult. Secondly, the fluorescence ratio, Fluo-3/Fura-Red, behaves non-linearly at Ca2+ concentrations less than approximately 500 nM and comparison with Fura-2-loaded single cell photometry studies suggests there is considerable amplitude distortion of the signal when the ratios are displayed on a linear scale. These problems may considerably limit the application of Fluo-3/Fura-Red ratiometric measurements.